In a two-way radio communications system, a set of mobile stations (MS) communicate with each other and/or a set of base stations. The MS may communicate with each other in “direct mode” in which simplex transmissions between two MS are carried out directly over a shared channel. Where the two MS, or an MS and a base station, are too far apart in order to communicate effectively over a shared direct mode channel, an intermediate station acts as a repeater by receiving and then re-transmitting the communications between the two end point MS on different channels. Traditionally, both the direct mode and repeater channels have been implemented using analog or digital frequency division multiple access (FDMA) two-way radio communications systems in which a single radio frequency (RF) carrier is allocated to each channel. Recently, the use of time division multiple access (TDMA) communication systems have been proposed for use in two-way radio communications systems, and in which a single RF carrier signal or medium is divided into time slots to carry a number of different channels.
Because the communication system carries many communications at one time, a MS may want to monitor other communications in the system. Scan is a feature that allows a MS to monitor other communications in the system. During a scan, a MS locks on to a specific RF carrier signal and inspects activity that may be present on the carrier signal. In a TDMA communications system, the MS must inspect activity on designated slots of the RF carrier frequency which are associated with the channel being scanned.
A priority scan involves scanning a higher priority channel whilst engaged in activity (e.g., a voice call) on a lower priority channel. The scan aims to determine whether any activity on the scanned channel is of interest, for example, whether a call on the scanned channel is addressed to the scanning MS. The scan involves switching away from the RF carrier signal of the active or in-use channel, on which the MS may be receiving a call, and switching to the RF carrier signal of the scan channel. This time away from the call that the MS is engaged in on the active channel results in the MS missing some of the active channel call resulting in an “audio hole”. If the scanned channel is carrying a communication that is not addressed to the MS or the scanned channel has no communications at all, then the MS returns to the call that it was previously engaged in and listening to on the active channel.
In order to reduce the size of the audio hole, the MS records whether the last time the MS performed a scan of the scan channel, that a communication on this scan channel was not addressed to the MS. Thus, the next time that the MS performs a scan and listens to the same scan channel, if the RF carrier signal associated with the scan channel is present, then the MS assumes that the activity or call on the scan channel is the same continuing communication which is addressed to the MS and quickly returns to the call that it is listening to on the active channel. This assumption that current RF carrier signal activity is a continuation of the RF carrier signal activity on the last scan enables the MS to reduce the size of the audio hole on the current scan.
However, while this assumption is practical for an analog or digital FDMA scan channel, it is not a good assumption for TDMA systems, since the fact that a RF carrier signal associated with the scan channel is present may be attributed to a new call (that may be addressed to the MS) in a different time slot as opposed to the same call (that is not addressed to the MS) in the same time slot. Thus, to determine whether the presence of the RF carrier associated with the scan channel is of a new call or the same call, the MS needs to synchronize and decode the RF carrier signal to determine whether the RF carrier signal is carrying a communication that is of interest to the MS. This results in a longer scan time for TDMA scan channels, and therefore causes the “audio hole” problem to be compounded in TDMA systems.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.